1. Field of the invention
The present invention relates to an apparatus and method for internal combustion engine control capable of preventing the decrease in purifying function of an exhaust purifying catalytic device.
2. Description of the Related Art
There is known a method for improving the fuel economy or other performance of an internal combustion engine by controlling the air-fuel ratio to a target value (for example, 22) leaner than the theoretical air-fuel ratio (14.7) to thereby carry out lean-combustion in the engine when the engine is driven in a predetermined driving state. However, if a three-way catalytic converter is used for the engine to which the aforesaid method is applied, nitrogen oxides (NOx) cannot be sufficiently purified during the lean-combustion since the three-way catalytic converter does not operate at its full potential in the lean air-fuel ratio range. In this respect, attempts have been made to reduce the emission of NOx even in the lean-combustion operation by using a so-called NOx catalyst, which adsorbs NOx discharged from the engine in an oxygen enriched state (oxidizing atmosphere) and deoxidizes the adsorbed NOx in a hydrocarbon (HC) excessive state (reducing atmosphere).
However, there is a limit to the amount of NOx which can be adsorbed by the NOx catalyst. If the engine is continuously driven in a lean-combustion mode, the catalyst will be saturated with NOx. In this case, most part of NOx gas discharged from the engine is emitted to the atmosphere. To obviate this, before or when the NOx catalyst is saturated with adsorbed NOx, a shift is made to the rich mixture control, which controls the air-fuel ratio to a theoretical ratio or its near values to thereby start the theoretical ratio operation or rich-combustion operation of the engine. The resultant exhaust gas containing plenty of unburnt gases creates a reducing atmosphere for deoxidization of NOx around the catalyst.
Regarding the timing at which the lean-combustion operation is switched to the theoretical ratio operation or rich-combustion operation, a method is known from Japanese Patent Application KOKAI Publication No. H5-133260 in which the elapsed time from the start of lean air-fuel ratio control is measured, and the changeover to rich air-fuel control is forcedly carried out when a predetermined time has elapsed. With this method, the lean air-fuel ratio control is started again upon completion of deoxidization of NOx adsorbed by the catalyst during the rich air-fuel ratio control. In this way, the lean-combustion and rich-combustion are alternately effected to reduce the emission of NOx.
According to the aforementioned related art, the purifying ability of NOx catalyst can be restored by deoxidizing and removing NOx adsorbed by the NOx catalyst. However, to the NOx catalyst adheres a substance which reduces the NOx adsorbing capability of the catalyst by adhering to a place where NOx is naturally adsorbed (hereinafter called a purifying capability reducing substance) such as sulfur and the compounds thereof.
The purifying capability reducing substances other than NOx cannot be removed even if the air-fuel ratio control as disclosed in Japanese Patent Application KOKAI Publication No. H5-133260 is effected, and the amount of adherence increases as the engine driving time elapses. If the adherence of purifying capability reducing substances is left as it is, the NOx adsorbing ability of the NOx catalyst is decreased, by which the purifying function thereof cannot be fulfilled sufficiently.
Japanese Patent Application KOKAI Publication No. H6-66129 discloses a technique in which an exhaust purifying catalytic device is heated by using an electric heater when it is judged that the amount of sulfur oxides adsorbed by the exhaust purifying catalytic device has reached a predetermined amount, by which the sulfur oxides adsorbed by the catalytic device is removed from the catalytic device. In this related art, however, an electric heater for heating the exhaust purifying catalytic device must be newly installed, and in addition electric energy must be supplied to the electric heater. The capacity of the battery mounted on a vehicle which is driven by an internal combustion engine is limited, and a battery of a high capacity must be used to heat the exhaust purifying catalytic device by using an electric heater.
Japanese Patent Application KOKAI Publication No. H5-76771 discloses a technique in which sulfur oxides adsorbed by an exhaust purifying catalytic device of a type which directly decomposes nitrogen oxides are oxidized or deoxidized when the engine is hot, by which sulfur oxides are removed from the catalytic device. In this related art, however, even if the adsorption of sulfur oxides becomes excessive, the sulfur oxides cannot be removed from the catalytic device when the engine temperature is low.